Magnesium alloy



Patented Nov. 7, 1939 UNITED STATES PATENT OFFICE MAGNESIUlW ALLOY NoDrawing. Original application November 23, 1936, Serial No. 112,288.Divided and this application August '7, 1939, Serial No. 288,771

3 Claims.

This invention relates to magnesium alloys and particularly to thosecontaining manesium in excess of approximately 80 per cent.

It is well known that the addition of certain metals to magnesiumresults in the production of alloys possessing good strengthcharacteristics, while the addition of other metals results in theproduction of alloys possessing good corrosion resistance. One of themajor problems of the magnesium industry is the preparation of an alloyin which both objectives are obtained at the same time. 7 r

Accordingly, the object of the present invention is to prepare megnesiumalloys which have improved physical properties and corrosion resistance.Other objects and advantages will appear as the description proceeds.

My invention is based on the discovery that highly desirable and usefulalloys can be prepared by the addition of silver to magnesium andmagnesium alloys, and that these alloys possess good strengthcharacteristics, combined with good corrosion resistance.

I have discovered that the strength characteristics of magnesium may beimproved to a very marked degree by the addition of suitable amounts ofsilver. The tensile strength of cast pure magnesium, for example, wasfound to be 14,000 pounds per square inch, while that of magnesium alloycontaining 2 per cent of silver was 21,100 pounds per square inch andthat of a magnesium alloy containing 4 per cent of silver was 22,300pounds per square inch. The yield strength of cast magnesium was 2,500pounds per square inch, while the yield strengths of the magnesiumalloys containing 2 per cent of silver and 4 per cent of silver were4,100 pounds per square inch and 5,100 pounds per square inchrespectively. The addition of 2 per cent and 4 per cent of silver topure magnesium raised the percentage. elongation of the cast metal from8.0 per cent up to 10.5 per cent and 10.0 per cent respectively. Thetoughness of the magnesium was likewise im-. proved by the addition ofsilver. The single blow impact value for pure magnesium was 9.0 footpounds, while the corresponding values for the magnesium alloyscontaining 2 per cent of silver and 4 per cent of silver were 12.6 and11.5 foot pounds respectively. The Brinell hardness values of these sametwo alloys were 34 and 37 respec; tively as compared with 33 for puremagnesium. As the percentage of silver in the alloy is increased, stillgreater improvements are obtained in the yield strength and hardness,with a pro portional decrease in the percentage elongation andtoughness. The magnesium alloy containing 8 per cent of silver, forexample, had a Brinell hardnessvalue of 45 and the magnesium alloycontaining 12 per cent'of silver had a Brinell hardness value of 51. 5Magnesium-silver alloys, particularly those containing approximately 4per cent or more of silver, are amenable to heat treatment. A solutionheat treatment of 18 hours at 770 F. increased the tensile strength ofthe magnesium alloy containing 4 per cent of silver from 22,300 poundsper square inch to 24,900 pounds per square inch, and increased thepercentage elongation from 10.0 per cent to 11.5 per cent withoutappreciably afiecting other properties. A subsequent precipitation heattreatment of 48 hours at 350 F. produced a small increase in the tensilestrength. Precipitation heat treatment; however, produced markedproperty improvements, particularly in tensile strength, yield strength,and hardness in alloys containing higher percentages of silver. Asolution heat treatment of 18 hours at 770 F. followed by aprecipitation heat treatment of 48 hours at 350 F., for example,increased the Brinell hardness of the magnesium alloy containing 8 percent of silver. from 45.0 to 55.5 and the Brinell hardness value of themagnesium alloy containing 12 per cent of silver from 51.0 to 65.5.Although beneficial property improvements are obtained in alloyscontaining approximately 0.3 to 15 per cent of silver, I normally preferto use from 0.5 to 8 per cent of silver, except in those cases wheremaximum properties in the heat treated condition are required, and thenI prefer to use alloys containing approximately 4 per cent to 8 per centof silver. Alloys with low percentages of silver are better adapted forplastic deformation operations, while allows with higher percentages ofsilver are better suited for the production of castings.

Furthermore, I have discovered that the addition of silver to commercialmagnesium is beneficial from the standpoint of corrosion resistance.

-This may be illustrated by alternate immersion corrosion testsconducted in a 3 per cent salt solution. At the end of 24 hours, puremagnesium had lost weight" at the rate of 65.8 mg/cm /day, while themagnesium alloys containingz per cent of silver and 4 per cent of silverlost only 51.2 and 17.3 mg/cm /day respectively. In another test theaddition of 1 per cent of silver produced 'a 30 per cent decrease in thecorrosion rate of magnesium.

I have also discovered that the beneficial effects of silver areretained when at least one of the metals aluminum, manganese, zinc isadded to magnesium-silver alloys, or, in other words, when silver isadded to an alloy consisting of magnesium with at least one of themetals aluminum, manganese, zinc. In such alloys, the percentage ofsilver may vary from about 0.3 per cent to 12 per cent, the percentageof aluminum may vary from about 0.5 per cent to 12 per cent, thepercentage of zinc may vary from about 0.5 per cent to 8 per cent, andthe percentage of manganese may vary from about 0.1 per cent to 1 percent (0.1 per cent to 0.5 per cent in alloys likewise containingaluminum), but the total percentage of added metals should not exceedapproximately 20 percent. The absolute percentage of each metal isdependent upon the use for which the alloy is intended and upon thepercentages of the other alloying ingredients. For plastic deformationprocesses, I normally prefer from 0.5 per cent to 4 per cent of silver,from 0.5 per cent to 6 per' cent of aluminum, from 0.2 per cent to 0.8per cent of manganese, and from 0.5 per cent to 2.0 per cent of zinc,with a maximum of approximately 8 per cent of added 'metals Forcastings, I normally prefer from 0.5 per cent to 8 per cent of silver,from 5 percent to 10 per cent of aluminum, from 0.1 per cent to 0.4 percent of manganese, and from 1 per cent to 4 per cent of zinc, thepercentage of total added ingredients varying from approximately 8 percent to 12 per cent.

The 'following examples serve to illustrate the beneficial efiect ofsilver in this class of maga and to .magnesium-aluminum-manganese alloyshas been found to be particularly beneficial when nesium alloys asexpressed by improvements in physical-mechanical properties, Forexample, the ternary magnesium alloy containing 2 per cent of silver and0.2 per cent of manganese had a tensile strength of 23,000 poundsper,square inch as compared with 18,000 pounds per square inch for thebinary magnesium alloy containing 0.2 per cent of manganese'and 21,100pounds per square inch for the binary magnesium alloy containing 2 percent of silver. The yield strength of this ternarymagneslum-silver-manganese alloy was 4,400 pounds per square inch ascompared with 3,000 pounds per square inch for the binary magnesiumalloy containing 0.2 per cent of manganese and 4,100 pounds per squareinch for the binary magnesium alloy containing 2 per cent of silver.

The addition of silver to magnesium-aluminum the alloys are used for theproduction of heat treated castings? Under such conditions, com--parable physical property improvements are obtained in alloys with andwithout manganese, but otherwise of similar composition, although thepresence of manganese is desirable when the alloy is used for extrusionpurposes. The addition of 2 per cent of silver, for example, increasedthe tensile strength of a solution heat treated magnesium alloycontaining 8 per cent of aluminum and 0.2 per cent of manganese from34,000 pounds per square inch to 36,000 pounds per square inch and theyield strength of this same alloy from 11,000 pounds per square inch to12,000 pounds per square inch. Maximum improvement due to the presenceof silver in alloys containing aluminum and manganese occurs in the agedor precipitation heat treated alloys, and here the effeet isparticularly pronounced in the yield strength and hardness values. Forexample, the

addition of 2 per cent of silver to this same magours, and thisprecipitated manganese lowers the ductility and workability of thealloy. Magnesium-silver-zinc alloys, suitable for rolling, containapproximately 0.5 per cent to 3 per cent of silver and 0.5 per cent toI'per cent of zinc, the balance being magnesium. Magnesium-silverzincalloys, suitable for extrusion, contain approximately 0.5 per cent to 6'per cent of silver and 0.5 per cent to 5 per cent of zinc, the balancebeing magnesium. Where ease of subsequent deformation is not required,as, for example, in

extruded sections, I normally prefer to usemagnesium-silver-manganese-zinc alloys containing 0.5 per cent to 5 percent of silver, 0.1 per cent to 0.6 per cent of manganese, and 0.5 percent to 6 per cent of zinc. The properties of such alloys, particularlythose containing more than approximately 3 per cent of silver and 2 percent of zinc, may be further improved by heat treatment.

Although the magnesium-silver-zinc alloys may be used for theproduction'of castings, I generally prefer to use, for such purposes,alloys containing silver, aluminum, and zinc, or silver, aluminum, zinc,andmanganese.

Manganese is soluble in this type of alloy to the extent of a few tenthsof a per cent, and in such amounts has no appreciable eifect on physicalproperty improvement, although it does improve the corrosion resistanceof the alloy. The choice between the alloy with or without manganese,but otherwise of similar composition, depends largely onfthe use forwhich the product is intended. A good composition, selected from themore corrosion resistant type of alloy, contains 2 per cent of silver, 8per cent of aluminum, 0.2 per cent of manganese, and 3 per cent of zinc.This alloy, inthe solution heat treated condition, has

gation, '72 Brinell hardness, and a single-blow impact value 01.2.5 footpounds:

I have likewise discovered that the corrosion resistance of themagnesium-silver alloys may be improved'very materially 'by the additionof at least one of the metals aluminum, manganese, zinc. For example,the loss in weight in a magnesium alloy containing 2 per cent of silverwhen tested in 3 per cent salt solution was 51.2 mg/cm /day, whereas theaddition of 0.2 per cent of manganese reduced this corrosion rate to 4.9mg/cm /day. In another instance, the addition of 0.9 per cent ofmanganese to a binary magnesium alloy containing 1 per cent of silverreduced the corrosion rate from approximately mg/cm /day to 1.7 mg/cm/day. In a similar fashion, the addition of 8 per cent of aluminum plus0.2 per cent of manganese to a binary magnesium alloy containing 2 percent of silver reduced the corrosion rate from 51.2 mg/cm /day to 1.2mg/cm /day, while the addition of 8 per cent of aluminum plus 0.2 percent of manganese plus 3 per cent of zinc to the binary alloy containing2 per cent of silver reduced the corrosion rate to 0.6 mg/cm /day.

The above described alloys may be prepared by the well known methods ofalloying metals with magnesium, such as adding the respective alloyingingredients to a bath of molten metal protected from oxidation by acover of a fluid flux. The various alloying ingredients may be addedsingly or simultaneously, and are usually added as pure metals, exceptin those compositions containing both-aluminum and manganese, in whichcase these two metals are preferably added in the form of a. 90-10aluminum-manganese hardener.

This is a division of my co-pending application Serial No. 112,288 filedNovember 23, 1936.

Other modes of applying the principle of my invention may be employedinstead of those explained, change being made as regards the ingredientsand the steps herein disclosed, provided those stated by any of thefollowing claims or their equivalent be employed.

I particularly point out and distinctly claim as my invention:

1. A magnesium base alloy comprising from 0.3 to 12 per cent of silver,from 0.5 to 12 per cent of aluminum, and from 0.5 to 8 per cent of zinc,the balance being magnesium.

2. A magnesium base alloy comprising from 0.5 to 8 per cent of silver,from 5 to 10 per cent of aluminum, and from 1 to 4 per cent of zinc, thebalance being magnesium.

3. A magnesium base alloy comprising 5 per cent of silver, 3 per cent ofaluminum, and 3 per cent of zinc, the balance being magnesium.

JOHN A. GANN.

